1. Field of the Invention
The invention relates to a discharge lamp. The invention relates especially to a discharge lamp which is used as back light of a projection type projector device such as a liquid crystal display device, DLP® (digital light processor) (registered mark) using a DMD® (digital micromirror device) (registered mark) or the like.
2. Description of Related Art
In a projection type projector device, there is a demand for illumination of images onto a rectangular screen, uniformly, and moreover, with adequate color rendering. Therefore, it has been proposed that the light source be an ultra high pressure mercury lamp in which the mercury vapor pressure during operation is at least 150 atm. Such an ultra high pressure mercury lamp is described in Japanese Patent Application JP-A-2-148561 which corresponds to U.S. Pat. No. 5,109,181 and Japanese Patent Application JP-A-6-528301 which corresponds to U.S. Pat. No. 5,497,049.
FIG. 1 schematically shows the arrangement of the ultra high pressure mercury lamp. In the figure, an ultra high pressure mercury lamp 1 has an essentially spherical light emitting part 10 and cylindrical hermetically sealed portions 12 which border the two ends of the light emitting part 10 which is, for example, a silica glass bulb 11. The interior S of the light emitting part 11 is filled with at least 0.15 mg/mm3 of mercury and a halogen gas as the emission substances for carrying out the halogen cycle. In the interior S, the ends of the electrodes 2, 3 are disposed opposite each other. Metal foils 4 for power supply are inserted into the hermetically sealed portions 12; the ends of the foils are connected to the base parts of the electrodes 2, 3. Outer leads 5 which project to the outside from the hermetically sealed portions 12 are connected to the base parts of the metal foils 4.
In the ultra high pressure mercury lamp, since the pressure of the interior S is extremely high in operation, it is necessary to connect the silica glass comprising the hermetically sealed portions 12, the electrodes 2, 3 and the metal foils 4 for power supply securely to one another in the hermetically sealed portions 12 which border the two ends of the light emitting part 10. The reason for this is that a poor adhesive property leads to escape of the added gases or to crack formation. In the process of hermetic sealing of the hermetically sealed portions, for example, the silica glass is heated at a high temperature of 2000° C., and in this state, the tough silica glass is gradually contracted and the adhesive property of the hermetically sealed portions is improved.
However, if the silica glass is heated to an overly high temperature, the adhesive property of the silica glass with the electrodes 2, 3 or the metal foils 4 is increased. However, it was regarded as disadvantageous that, after completion of an ultra high pressure mercury lamp, the hermetically sealed portions 12 can be easily damaged.
This disadvantage is based on the fact that, in the stage of gradual temperature decrease of the hermetically sealed portions after heat treatment, due to the different coefficients of expansion between the tungsten comprising the electrodes 2, 3 and the silica glass comprising the hermetically sealed portions 12, the relative amounts of expansion differ; this causes crack formation in their contact regions. It can be imagined that these cracks are extremely small, but cause crack growth, the ultra high pressure state in lamp operation also playing a part, and that they cause damage to the ultra high pressure mercury lamp.
According to Japanese Patent Application JP-B-3670414 and corresponding U.S. Pat. No. 6,903,509, the above described crack formation is caused by a gap which inevitably forms in the region in which the metal foil is welded to the electrode in the hermetically sealed portion. Furthermore, Japanese Patent Application JP-B-3670414 and corresponding U.S. Pat. No. 6,903,509 describe that, by the area of the respective metal foil which is connected to the respective electrode having a smaller width than the width in the remaining area of the metal foil, the area with the smaller width wrapping at least partially around the outside surface of the electrode, the above described gap formation is avoided, and that, in this way, crack formation can be prevented, as is shown in FIGS. 8(a) to 8(c).
FIGS. 8(a) to 8(c) are schematics of an electrode-mount assembly and of the metal foil of the conventional ultra high pressure mercury lamp. FIG. 8(a) shows the electrode end mount assembly in a front view. FIG. 8(b) shows a view in which the metal foil is viewed from overhead. FIG. 8(c) are cross sections taken along lines A-A and B-B in FIG. 8(b).
In FIGS. 8(a) to 8(c), the metal foil 4′ has a region 41′ with a small width with a groove-like overall shape which is connected to the base part 22A′ of the upholding part of the electrode 22′ and a wide region 42′ with a cross section in the direction of width which is formed to be Ω-shaped by formation of a groove part 46′ with a width and depth which are uniform over the entire length, and which borders the region 41′ with a small width. Because the electrode end mount assembly 20′ which has been produced by connecting the outer lead 5′ and the base part 22A′ of the upholding part of the electrode 22′ with such a metal foil 4′ is inserted and hermetically sealed in a silica glass tube which constitutes the bulb, an unwanted gap is reliably prevented from being formed between the base part 22A′ of the upholding part of the electrode 22′ and the metal foil 4′. This should mean that crack formation in the hermetically sealed portion is thus prevented.
However, crack formation in the hermetically sealed portion could not be completely prevented by the technology described in Japanese Patent Application JP-B-3670414 and corresponding U.S. Pat. No. 6,903,509 for the reason described below.
FIGS. 9(a) to 9(c) show schematics describing the disadvantage in production of an ultra high pressure mercury lamp using the electrode end mount assembly 20′ which had been produced according to FIGS. 8(a) to 8(c). In FIGS. 9(a) to 9(c), the silica glass tube 10′ is omitted on the side into which the electrode end mount assembly 20′ is not inserted.
As shown in FIG. 9(a), for the electrode end mount assembly 20′, the upholding part of the electrode 22′ and the component 50′ for the outer lead are connected to the metal foil 4′. On the base side of the component 50′ for the outer lead, an elastic connection strip R′ is attached. When this electrode end mount assembly 20′ is inserted into the silica glass tube 10′, due to factors such as the skill of the operator and the like, there are cases in which the electrode end mount assembly 20′ is arranged inclined with respect to the center axis of the silica glass tube 10′, as shown in FIG. 9(a). In this case, the upholding part of the electrode 22′ is arranged eccentrically from the center axis of the silica glass tube 10′.
In this state, if a shrink seal is attempted in which, for example, the glass is heated from the outside of the silica glass tube 10′, for example, by means of a burner or the like, in order to allow the upholding part of the electrode 22′ and the metal foil 4′ which has been inserted into the silica glass tube 10′ to adhere hermetically to the glass, as is shown in FIG. 9(b), as is described below, cracks form in the silica glass of the hermetically sealed portion.
In shrink sealing, since the heating force of the burner and the burn time are fixed under certain conditions, the glass is uniformly contracted in the direction to the center axis of the silica glass tube 10′. In shrink sealing, a force is applied for moving the upholding part of the electrode 22′ in the direction to the center axis of the silica glass tube 10′ when the molten glass reaches the surface 22X of the upholding part of the electrode 22′ on the side which is adjacent to the inside wall of the silica glass tube 10′.
In the component 50′ of the outer lead, movement in the direction which orthogonally intersects the center axis is controlled by the elasticity of the connection strip R′. The cross section of the wide part 42′ of the metal foil 4′ is made Ω-like over the entire length by the component arrangement shown above using FIGS. 8(a) to 8(c). The bending strength of the wide part 42′ is high. Therefore, since the above described force for moving the upholding part of the electrode 22 in the direction toward the center axis of the silica glass tube 10′ is concentrated on the region with a small width 41′ in which the bending strength is weakest and which is not connected to the base part 22A′ of the upholding part of the electrode 22′, the region with a small width 41′ of the metal foil 4′ bends to a great extent, as is shown in FIG. 9(b). The adhesive property of the bent region of the metal foil 4′ on the glass is thus weakened. Thus, in operation of the completed ultra high pressure mercury lamp shown in FIG. 9(c), cracks form by application of a mercury vapor pressure in the interior S on the hermetically sealed portion 12′.
When the outside diameter of the base part 22A′ of the upholding part of the electrode 22′ differs from the outside diameter of the outer lead 5′, the following disadvantages arise when the electrode mount assembly 20′ is produced using the metal foil 4′ in which the width and depth of the groove 46′ correspond to FIGS. 8(a) to 8(c).
It is necessary to fix the width and depth of the groove 46′ according to the outside diameter of the outer lead 5′ or the outside diameter of the upholding part of the electrode 22′. However, since normally the outer lead 5′ is thicker than the upholding part of the electrode 22′, in the case of construction of the width and depth of the groove 46′ according to the outside diameter of the upholding part of the electrode 22′, the outer lead 5′ cannot be accommodated in the groove 46′. If the attempt is made to accommodate the outer lead 5′ in the groove 46′ with such a construction by force, there is the danger that the metal foil 4′ will be damaged.
In the case of construction of the width and depth of the groove 46′corresponding to the outside diameter of the outer lead 5′ which is thicker than the upholding part of the electrode 22′, both the upholding part of the electrode 22′ and also the outer lead 5′ which is held in the groove 46′ can be connected to the groove 46′. However, in the electrode mount assembly 20′, the center axis of the upholding part of the electrode 22′ does not agree with the center axis of the outer lead 5′, by which the upholding part of the electrode 22′ deviates eccentrically from the outer lead 5′.
In this state, in which the upholding part of the electrode 22′ deviates eccentrically from the outer lead 5′ which is being inserted into the silica glass tube 10′ with the electrode mount assembly 20′ which is connected to the metal foil 4′, since the inside diameter of the silica glass tube 10 is constructed according to the outside diameter of the outer lead 5′ with a large diameter, the upholding part of the electrode 22 is in the state in which it deviates eccentrically from the center axis of the silica glass tube 10′, as is shown in FIG. 10(a). In this state, when shrink sealing is performed, as was described above, based on the Ω-shaped execution of the cross section of the wide region 42′ of the metal foil 4′ over the entire length, the above described force for moving the upholding part of the electrode 22′ in the direction of the center axis of the silica glass tube 10′ is concentrated on the region with the small width 41′ with an extremely low bending strength. As a result, the region with a small width 41 of the metal foil 4′ bends greatly, as is shown in FIG. 10(b). The adhesive property of the bending site of the metal foil 4′ on the glass is weakened. Thus, the disadvantage arises that, by applying a high mercury vapor pressure of the interior S in operation, cracks form in the hermetically sealed portion 12′.
As was described above, in a conventional ultra high pressure mercury lamp, it is regarded as disadvantageous that, by bending the region 41′ of the metal foil 4′ with a small width, the adhesive property of the region 41′ with a small width near the interior S of the ultra high pressure mercury lamp on the glass in the vicinity is adversely affected, by which cracks form in the hermetically sealed portion 12′.